Fillers such as silicas and carbon blacks are conventionally added to rubber components such as natural rubbers and styrene butadiene rubbers to enhance mechanical strength. Such rubber compositions are widely used for tires requiring abrasion resistance and mechanical strength. Because rubber compositions including fillers exhibit an increased viscosity during the kneading, rolling and extrusion of the rubbers, plasticizers such as process oils are used in order to improve the processability and the flowability.
Even if properties such as mechanical strength and hardness are appropriate as the compositions are just produced, the rubbers used in applications such as tires degrade their performance during long use. This problem is caused by the migration of additives such as plasticizers to the surface of the rubbers.
One approach to preventing the migration of additives such as plasticizers is to add liquid diene rubbers to rubber compositions in place of conventional plasticizers such as process oils. Rubber compositions prepared in this manner attain excellent processability, and crosslinked products thereof are advantageously prevented from the migration of the components after crosslinking (see, for example, Patent Literatures 1 and 2).
However, rubber compositions containing liquid diene rubbers sometimes have insufficient filler dispersibility. Further, crosslinked products of such rubber compositions do not necessarily have sufficient properties such as mechanical strength and abrasion resistance at all times. Furthermore, crosslinked products of such rubber compositions, in particular, tires are demanded to achieve further improvements not only in mechanical strength such as tensile strength but also in rolling resistance performance. In addition, it is desired that such tires overcome the general difficulty in achieving simultaneous enhancements in steering stability and ice grip performance.